Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars

Lauren Sumner-Rooney, Imran Rahman, Julia Sigwart, Esther Ullrich-Lueter

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Abstract

Photoreception and vision are fundamental aspects of animal sensory biology and ecology, but important gaps remain in our understanding of these processes in many species. The colour-changing brittle star Ophiocoma wendtii is iconic in vision research, speculatively possessing a unique whole-body visual system that incorporates information from nerve bundles underlying thousands of crystalline ‘microlenses’. The hypothesis that these might form a sophisticated compound eye-like system regulated by chromatophores has been extensively reiterated, with investigations into biomimetic optics and similar supposedly ‘visual’ structures in living and fossil taxa. However, no photoreceptors or visual behaviours have ever been identified. We present the first evidence of photoreceptor networks in three Ophiocoma species, both with andwithout microlenses and colour-changing behaviour. High-resolution microscopy, immunohistochemistry and synchrotron tomography demonstrate that putative photoreceptors cover the animals’ oral, lateral and aboral surfaces, but are absent at the hypothesized focal points of the microlenses. The structural optics of these crystal ‘lenses’ are an exaptation and do not fulfil any apparent visual role. This contradicts previous studies, yet the photoreceptor network in Ophiocoma appears even morewidespread than previously anticipated, both taxonomically and anatomically.
Original languageEnglish
Article number20172590
JournalProceedings of the Royal Society of London. Series B, Biological Sciences
Volume285
Issue number1871
Early online date24 Jan 2018
DOIs
Publication statusPublished - 31 Jan 2018

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